Lithium secondary batteries have high voltage and high capacity, and thus have been widely used as power supplies of mobile phones, digital cameras, camcorders, notebook computers, electric vehicles, and so on. The lithium secondary batteries, which are commonly circulated, use a liquid electrolyte in which an electrolytic salt as an electrolyte is dissolved in a non-aqueous solvent. Since many of non-aqueous solvents are flammable, there have been demands to secure safety of the lithium secondary batteries.
For the purpose of securing safety, all-solid lithium secondary batteries have been proposed that use, instead of the non-aqueous solvent, a so-called solid electrolyte made of solid materials. A positive electrode of the battery contains various components, such as a positive electrode active material, an electrical conducting material, and the electrolyte. Among these components, Li2S used as the positive electrode active material is drawing attention because of its high theoretical capacity.
Li2S itself is too low in ionic conductivity to be used as the positive electrode active material, and thereby needs to be used with the conductive component. Therefore, the inventors of the present invention had proposed a positive electrode containing Li2S, acetylene black as an electrical conducting material, and Li2S.P2S5 as a solid electrolyte (Abstracts of Presentations at the 53rd Symposium on Batteries in Japan. 2012. p. 555: Non-Patent Literature 1).